1. Field of the Invention
The present invention relates to a quantum memory that enables transfer of quantum information, and an information processing method using the quantum memory.
2. Description of the Related Art
Quantum information processing techniques such as quantum calculations and quantum communications use the quantum state of a physical system to represent information so as to process the information by manipulating the quantum state. With these techniques, a physical system and structure suitable for manipulation of the quantum state is generally different from a physical system and structure suitable for storage of quantum information. Thus, the need for a quantum memory intended to store quantum information has been recognized. Furthermore, in quantum communications, a quantum memory is required to store quantum information carried by the quantum state of light, as the quantum state of a stationary physical system, and to reproduce and utilize the stored information as required.
Such a quantum memory has the following requirements (1) to (3). (1) The quantum memory must be able to retain the quantum state for a time sufficient to process information. (2) The quantum memory must appropriately resist noise. (3) The quantum memory must be able freely transfer quantum information in a simple manner between arbitrary memory cells in the memory assigned for storage or between such a memory cell in the memory and a physical system expressing a quantum bit in a gate executing calculations. However, there have not been any quantum memories that meet these requirements.
In particular, in connection with the transfer of quantum information, i.e., the quantum state referred to in the above item (3), it is impossible in principle to perform an operation of reading complete quantum information on a general quantum state and then writing it to a different quantum bit. On the other hand, the quantum state can be transferred utilizing a method called “quantum teleportation” which transmits the whole quantum state to a different physical system without reading information on the quantum state. However, there have not been any quantum memories that can carry out teleportation between arbitrary quantum bits using a practical method. To efficiently process quantum information, it is important to transfer quantum information within a quantum memory or to temporarily transfer quantum information from a calculation section (gate section) to a memory section and to return the quantum information to the calculation section as required.
In recent years, a method has been proposed which generates entanglement (a quantum-mechanically entangled state) between atomic ensembles simply by allowing classical laser light (coherent light) to act on the atomic ensembles and observing the polarization of transmitted laser light (Lu-Ming Duan et al., Phys. Rev. Lett., 85(26), 5643 (2000)). In September 2001, the results of experiments were disclosed which verify a method almost equivalent to the above method (B. Julsgaard et al., Nature 413, 400 (2000)).
With the method of Duan et al., entanglement is generated for atomic ensembles 1 and 2 by irradiating the atomic ensembles 1 and 2 with classical laser light so that the light pierces the ensembles and subjecting the transmitted light to homodyne detection. This method generates entanglement utilizing the quantum state of atomic ensembles instead of a single atom or photon. This method thus advantageously resists noise. Furthermore, it can utilize atoms in a free space and thus does not require any high-performance optical resonators. It can also generate entanglement using classical intense laser light. Thus, this is a simple, convenient, and practical method for generating entanglement.
The entanglement generation is an important technique that plays a key role in quantum teleportation. Accordingly, if the entanglement generating method and quantum teleportation utilizing the method could be utilized to freely transfer quantum information within a quantum memory, they are expected to constitute simple, convenient, practical, and excellent means for transferring quantum information. However, no methods have been known which utilize the entanglement generation based on the above method and quantum teleportation to freely transfer quantum information within a quantum memory.